MONTREAL — Advocates for five men arrested under security certificates said they were stunned to learn that Canada’s spy agency believed cases against them could fall apart if it could not use information obtained by torture.

On Saturday, the Montreal Gazette revealed that in 2008, the Canadian Security Intelligence Service (CSIS) warned the minister of public security that it could become impossible to use security certificates to arrest and deport suspected terrorists if it was prohibited from using information from foreign regimes known to use torture.

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In a letter obtained by the Montreal Gazette, former CSIS director Jim Judd warned that a proposed bill then before Parliament “could render unsustainable the current security certificate proceedings.” A security certificate is a means by which the government may detain and deport non-citizens perceived as a threat to national security.

The letter calls into question CSIS’s assurances that it did not countenance the use of torture abroad.

On Sunday, an advocacy group representing men who have been detained under security certificates hailed the report as proof that CSIS and top government officials knew terror cases might not stand up without information obtained under duress.

“It’s very disturbing that CSIS is so dependent on information that’s obtained from the use of torture that they would consider that these cases would fall apart without it,” said Mary Foster, a member of the Coalition Justice for Adil Charkaoui.

Charkaoui, 37, was arrested in 2003 under a security certificate and cleared in 2009. He is seeking compensation from the federal government and has led a campaign against the arbitrary arrest of terror suspects.

“It is unbelievable. CSIS has been lying to us for years,” Charkaoui said in a statement. Charkaoui charged that by signing security certificates against him and other suspects after he received the warning from Judd, then-public safety minister Stockwell Day was “effectively condoning the use of torture and condemning us to several more years of arbitrary detention.”

Hassan Almrei, another former terror suspect cleared of allegations in 2009, also hailed the revelation. “It really makes me sick to think that when I was sitting in solitary confinement on secret allegations for almost eight years, the head of CSIS knew that my case, and the cases of the other men held on security certificate, were completely baseless, because they were likely based on information that came from torture,” the Toronto man said in a statement.

In February 2007, the Supreme Court struck down the security certificate legislation as unconstitutional and the government passed a new law to replace it in 2008.

In his letter, Judd urged the minister to fight a Liberal amendment to prohibit CSIS and the courts from using information obtained by torture or “derivative information” — information initially obtained by torture but subsequently corroborated by legal means.

But despite Judd’s urging, the government passed the amended Bill C-3 in February 2008.

However, Foster said the letter reveals an alarming readiness to use information that might have been obtained by torture despite the legal ban on doing so. “CSIS on their own evaluation thought that these cases were built on information that the law said they couldn’t use,” she said.

“It’s disturbing that they then went ahead and advised the ministers to issue the certificates. And it’s disturbing that the ministers went ahead and signed it, knowing that CSIS believed that it was based on information that would not pass (scrutiny),” she added.

In addition to Charkaoui and Almrei, the coalition is also demanding that the government drop charges against Toronto men Mahmoud Jaballah and Mohammad Mahjoub as well as Mohamed Harkat of Ottawa.

In an email message on Friday, CSIS spokeswoman Tahera Mufti said the agency opposes the mistreatment of any individual by any foreign entity. “We do not condone the use of torture or other unlawful methods in responding to terrorism and other threats to national security,” she wrote.